If you are asking me ... it's whatever terms were agreed upon when setting up the trade.

On low value material, I don't think insurance is necessary and is probably wasteful. If we're talking current slot cards for example, which generally speaking cost the seller nothing ... then the loss isn't that significant to worry about.

Items of value should be insured between parties who don't know each other, or who have not consumated trades previously. Who pays for the insurance should be agreed upon before mailing.